Color pick-up tube with circuit for minimizing cross-talk



April 30, 1963 I L. HEIJNE ETAL 3,087,985

COLOR PICK-UP TUBE WITH CIRCUIT FOR MINIMIZING CROSS-TALK Filed Jan. 26. 1959 2 Sheets-'Sheet 1 INVENTORS Leo PoLn Helme 2 ANToNlus JOHANNES ne RooY sJoeno THsuNls snp BY MEW AGEN April 30, 1963 1 HEIJNE ETAL 3,087,985 coLoR PICK-UP TUBE WITH CIRCUIT FOR MTNTMTZTNG CROSS-TALK Filed Jan. 2e. 1959 2 Sheets-Sheet 2 @HSS 127 ./INVENTORS FOLD HEIJNE ANTONIUS JOHANNES DE ROOY SJOERD THEUNIS STAP LEO 3,087,935 COLR PCK-Ul TUBE WHTH CERCUHT FR MNllt/IEZNG CROSSJIALK Leopold Heine, Antonius ohannes De Rooy, and Sioerd Theunis Stap, all of Eindhoven, Netherlands, assigner-s to North American Philips Company inc., New York, N.Y., a corporation of Delaware Filed Jan. 26, i959, Ser. No. 789A@ Claims priority, application Netherlands l'an. 3l, w53 17 Claims. (Cl. 17E-*5.4)

The invention relates to a device for converting radiation images into colour component signals. The device comprises a camera tube having a radiation-sensitive target plate which is adapted to be scanned by an electron eam. The target plate is made of semi-conductive material and is provided with a plurality of alternating electrode elements united in electrically different groups. The target sections in the tube associated with the different groups of elements have either different spectral sensitivity or are irradiated by a radiation of different spectral ranges. The tube has at least one net-shaped electrode arranged in front of the target plate on the scan side thereof.

It is known to obtain different colour component signals in a television recording system comprising a single camera tube by dividing the electrode of the target plate of the camera tube into a plurality of elements, which are united in electrically different groups. By using a complex optical filter which is subdivided in accordance with the pattern of the electrode elements or by using photo-conductive material in the target plate, having different spectral sensitivity at the areas of the elements ofy different groups, each group of electrode elements can produce an electrical signal which corresponds to a particular colour component of the radiation image projected onto the target plate.

With a known device of this kind the different colour component signals are derived each from the associated group of electrode elements. This gives rise to the disadvantage that owing to capacitative coupling between the elements of the different groups the signal voltage of a given group of electrode elements contains not only the signal of the colour component associated with this group of elements, but also a voltage associated with the other colour components. It is not a simple problem to separate the signal voltages derived from the different groups of electrodes to obtain separate colour component signals. It has therefore been suggested to connect the different groups of electrode elements each to a different high-frequency voltage and to derive the separate colour component signals from a signal provided at the collecting electrode by the return beam (which beam is `formed by the electrons returning from the target plate in the acceleration part of the camera tube) by means of electrical separating filters tuned to different frequencies.

The object of the invention is also to obtain separate colour component signals, but in a manner simpler than that known hitherto. It is based on the recognition of the fact that the object aimed at can be attained not by stabilizing, as is suually done, the whole surface of the target plate on the same potential, but by stabilizing the different parts of the target plate associated with the elements of different electrode-element groups representing each a given colour component on different potentials or in different directions.

In accordance with the invention a device of the abovementioned kind is provided in which the first, net-shaped electrode arranged nearer the target plate has a positive bias voltage relative to the cathode of the tube. This voltage exceeds the first emission-equilibrium potential of the material at the target-plate surface scanned by the electron beam. The electrode elements are divided into therese Patented Apr. 3l), 1963 two or three groups, which have different positive voltages relative to the cathode of the tube. Not more than one group of electrode elements has a bias voltage lower than the first equilibrium Voltage, and not more than one group has a bias voltage which is higher than the first equilibrium voltage but lower than the bias Voltage of the said first net-shaped electrode. Further, not more than one group has a bias voltage which is higher than the bias Voltage of this first net-shaped electrode. lf the bias voltage of one group of electrode elements is lower than the first equilibrium voltage and if the bias voltage of a second group of electrode elements is higher than the bias voltage of the said first net-shaped electrode, a secV ond net-shaped electrode is arranged farther away from the target plate than the first net-,shaped electrode, and is at a positive bias voltage lower than the bias voltage of the first net-shaped electrode.

The term first emission equilibrium voltage of a Ina-f terial is to be understood to mean herein the minimum voltage up to which electrons bombarding this material are to be accelerated to attain a secondary-emission coefficient 6:1. As is known, the curve indicating the secondary-emission coefiicient of a material as a function ofthe velocity (voltage) of the electrons striking this material has two points of intersection with the line 6:1 i.e. a first point of intersection at an ascending portion and a second point of intersection at a descending portion of the curve. The acceleration voltage of the electrons associated with the first point of intersection is the aforesaid rst emission-equilibrium voltage, briefly referred to as the equilibrium voltage.

With the device according to the invention the scanned side of those target sections on which are located the electrode elements of which the voltage, relative to the cath-y ode, is lower than the equilibrium voltage of the material of the scanned target surface is stabilized on cathode potential. Those target sections on which are located the electrode elements of which the voltage exceeds the said equilibrium voltage, are stabilized on the voltage of they first net-shaped electrode. If the voltage of the lastmentioned electrode elements is lower than the voltage at the first net-shaped electrode, this stabilization takes place yby secondary emission, the secondary electrons being collected either `by the first net-shaped electrodey or by a collecting electrode for the return beam in the camera tube.

If, on the contrary, the voltage of the last-mentioned electrode elements exceeds the voltage at the first neteshaped electrode, this stabilization takes place by capturing electrons from the scanning beam. Since the signal voltages obtained at the first two stabilizations have opposite polar' ities, the associated colour component signals can be read ily derived by detection in opposite sense from a complex signal containing both of them. Such a complex signalv may be obtained, for example, from the electrode ele` ments concerned in common.

Complex signals, produced at the stabilization of target' sections on cathode potential and of other target sections shaped electrode, for instance equal to the aforesaid equilibrium voltage. Those colour component signals which are obtained by stabilization on the potential of the first net-shaped electrode can then be taken from this electrode,"

whereas the colour component signal due to the stabilization of the target sections concerned on cathode potentiall can be obtained from a collecting electrode for the return beam.

l Although with the device according to the invention the target plate in the camera tube will, as a rule, be made of photo-conductive material, it may be made, as an alternative, from a transparent, non-photosensitive semi-conductor, for example, conductive glass, i.e. a kind of glass having an electrical resistivity of 109 to l012 ohm. cm. In this case the target plate must be provided on the scanning side with a radiation-sensitive mosaic capableV of emitting photo-electrons. Since these photo-electrons must be conducted away, all electrode elements must be at a voltage which is lower than the voltage at the rst net-shaped electrode. In a device according to the invention comprising a camera tube with such a target plate, the electrode elements thereof are united in not more than two groups. The lirst of these groups has a voltage which is lower than .the equilibrium voltage of the light-sensitive mosaic, whereas the second group is at a voltage intermediate between the equilibrium voltage and the voltage of the first net-shaped electrode. A second net-shaped electrode has, relative to the cathode, a positive bias voltage which is lower than the bias voltage of the group of electrode elements which is at the higher potential.

With a device according to the invention `the voltage of the first net-shaped electrode is preferably adjusted to about double the iirst emission-equilibrium voltage of the material on the scanning side of the target plate.

The invention will now be described with reference to two embodiments shown in the drawing.

FIG. l illustrates diagrammatically one embodiment for producing two colour component signals with the use of a camera tube having a photo-conductive target plate.

FIG. 2 illustrates diagrammatically a second example for producing two colour component signals with the use of a camera tube having a target plate of conductive glass.

FIG. 3 illustrates a device according to the invention for producing three colour component signals and FIG. 4 shows, in a front view, an example of the disposition of the electrode elements in the camera tube in the device shown in FIG. 3 and FIG. 5 shows a slightly modified arrangement of these electrode elements. p

The device shown in FIG. 1 has a camera tube 1 with, at one end, an electron gun 2 with a cathode 3 and, at the other end, near the image window 4, a photo-conductive target plate 5. The anode 6 of the electron gun 2 is electrically connected -to a conductive wall coating 8, screened, on the side of the target plate by a gauze-shaped electrode 7, which is pervious to electrons. The tube 1 is surrounded by the conventional deflection and focusing coils, which are illustrated in the drawing diagrammatically by the single coil 9. At a small distance from the target plate 5, on its scanning side, provision is made of a net-shaped electrode 10, which is connected to a supply conductor 11 leading outside the tube.

On the inner side of the image window 4 is arranged a two-colour filter consisting of alternate paths 12 and 13, which paths 12 and 13 have different spectral transparency, the paths 12 being for example mainly pervious to red and the paths 13 mainly to green. On each of these paths provision is made of a narrow, line-shaped electrode element, the elements 14 being located on the paths 12 and being electrically connected to a supply conductor 16 and the elements on the paths 13 being electrically connected to a supply conductor 17. These connections are established, for example, by passing over the ends of the electrode elements, two conductive paths, each of which is connected only to the elements 14 and 15 respectively for direct currents. The electrode elements 14 and 15 must be pervious to light; they may be made, for example, of a very thin metal layer or of conductive tin oxide.

On the electrode elements 14 and 15, the direction of length of which is transverse to the plane of the drawing, is arranged the target plate 5, consisting of a photoconductive semi-conductor. This target plate Vmay consist, for example, of photo-conductive selenium or antimony trisulfide. A very suitable material is, moreover, lead monoxide.

'Ihe cathode 3 of the electron gun 2 is connected to the negative terminal of a voltage source 19. As is indicated in FIG. l, the cathode 3 may 4be connected to earth, but this is not necessary. From the voltage source 19 are obtained various bias voltages, which are positive relative to the cathode 3 and which are supplied to the electrode elements 14 (bias voltage V1) the electrode elements 15 (bias voltage V2) and the net-shaped electrode `10- (bias voltage V5) to which end the conductOrs 16 and 17 are connected -via a signal resistor 22 and 21 respectively, and also the conductor 11, to different points of the said voltage source. These bias voltages are adjusted so that V1 E V2 V5, wherein E designates the -iirst emission equilibrium voltage of the material on the scanning side of the target plate 5, as deiined above. Preferably V1V5-V2 and V5 is adjusted approximately to 2E.

With a target plate of amorphous selenium, E is 20 to 30 v., approximately the same value is found with a target plate of lead monoxide, if at least this target plate has been exposed to moist air prior to the final exhaust of the tube. Without this exposure the lead monoxide would have a higher emission equilibrium voltage. The iirst emission equilibrium volta-ge may be iniiuenced by providing the target plate on the scanning side with a material having a high or a low emission equilibrium voltage. A high emission equilibrium voltage is obtained by applying, for example, a small quantity of silver or gold to the scanning side of the target plate by vaporisation, whereas a low emission equilibrium voltage may =be obtained, for example, by applying to the target plate a thin layer of a satisfactorily secondary-ernissive oxide or a halide of an alkali or an alkali-earth metal for example magnesium-oxide or -fluoride or cryolite.

Such an increase or a decrease of the iirst emission equilibrium voltage, if desired, need not be produced throughout the target surface to be scanned. As will be described hereinafter, the operation of the arangement is based on different stabilisations of the target sections opposite the electrode elements 14 and of the sections opposite the electrode elements 15. lIn the stabilisation of the inst-mentioned sections secondary emission is not desired, these sections are therefore preferably provided with a very thin layer of material with a high emission equilibrium voltage, for example silver or gold. The quantity thereof must, of course, be so small that the conduction along the target surface is substantially not aifected.

The last-mentioned target sections, i.e. those to be stabilized on the potential of the net-shaped electrode 10, are preferably provided with a material with satisfactory secondary emission, for example, cryolite.

Antimony trisulfide has, in itself, less favourable secondary-emission properties. With a target plate of this material the scanning side is therefore provided, opposite the electrode elements 15, with a small `quantity of a satisfactorily secondary emissive material.

With an emission equilibrium voltage vE of about 20 v. on the scanning side of the target plate 5 in FIG. l, the following values of the various bias voltages are of practical use: V1-l0 v., V2-30 v., V5-40` v. (positive with respect to the cathode of the tube).

The electrode elements 14 and 15 are connected, in common, via capacitors 22 and 23, to the input of a video amplifier 25. To the output of this amplifier are connected two detectors 26 and 27, serving as separation stages and allowing each only a signal over and below a given level respectively to pass. y

'Ihe device described above operates as follows. During the operation the side of the target plate 5 facing the electron gun 2 is scanned along lines transverse to the directions of the electrode elements 14 and 15 by an electron beam 28 emanating from the electron gun 2, the eifective diameter of which, transverse to the direcaos'aese tion of the electrode elements, is not larger than the Width of each of the colour paths 12 and 13 of the two colour filter provided on the image window. With this scan those parts of the surface scanned which are located opposite the electrode elements 14 are stabilized on the potential of the cathode 3, since the electrons striking these target sections have such a low velocity that in the case of secondary emission, the secondary emission coeicient is lower than l. The sections of the scanned surface opposite the electrode elements 15 are, on the contrary, stabilized on the potential of the netshaped electrode 16, since the potential of these sections of the target surface is always higher than the equilibrium Voltage E of the scanned surface, and is not lower than the bias voltage V2 of the electrode elements 1S. The electrons set free or returned by the target surface upon this stabilisation on the voltage V5 of the net-shaped electrode 1i) are collected, similar to the electrons returned upon the stabilisation of the target sections associated with the electrode elements 14-all of these electrons forming together the return beam 29-for the major part iby the anode 6 and the wall coating 8, connected thereto.

A radiation image, projected onto the target plate 5 via a diagrammatically shown, optical system 30, and subdivided by the colour filter consisting of the ycolour paths 12 and 13 into the two colours concerned, causes `a local increase in the electrical conductivity of the target plate in its `direction of thickness in accordance with the local intensity of the irradiation. Consequently, between two successive scans of the same point of the target plate the potential of this point changes and this the stronger the greater is the local intensity of the irradiation. This potential variation is such that the target sections stabilized on cathode potential attain a higher potential and hence 4become positive relative to the cathode, whereas the target sections stabilized on the potential of the net-shaped electrode attain a lower potential tand hence become negative relative .to the electrode 19. With the subsequent scan of the target plate, the various sections thereof are again stabilized on their initial potentials. The stabilization (on cathode potential) of the target sections associrated with the electrode elements 14 produces, across the signal resistor 29, -a negative signal voltage, Whereas the stabilization of the target sections associated with the electrede elements 15 on the potential of the electrode 1t? produces, across the signal resistor Ztl, a positive signal voltage. These signal voltages are both supplied .to the video amplifier 25. Since the two groups of electrode elements 14 and 15 are connected, in common, to the input of the `arnpliier 25, capacitative cross-talk between the elements of the two groups is unimportant. Owing to the opposite polarities the two signal voltages can be separated in a simple manner, subsequent to ampliiication, i.e. lby detection in opposite senses by means of the two separation stages 26 and 27. Thus two separate colour cornponent signals S1 and S2 lare obtained. rFhe signal S1 is associated with the colour component (in this case red) determined by the paths 12 and the signal S2 is associated with the colour component (in this case green) determined by the paths 13 of the two colour filter. The level with respect to which the output signal of the 'amplifier 25 is to be detected by the two separation stages 26 tand 2'7 in opposite senses, is given by the black level which occurs in each image period at the suppression of the electron beam 28 during the ily-back time. This level is therefore clearly `determined and the `correct detection to separate the two signals S1 4and S2 does not give rise to diliiculties.

The desired signals S1 and S2 do not only occur in the complex signal voltage to be obtained, in common, `from the electrode elements, but also, though each with opposite polarity, across a signal resistor 31, via which the anode 6 is connected to the positive terminal of a voltage source (not shown). The signal voltage `across this resistor is produced by the electrons returning from the target plate 5, i.e. by the return beam 29. Instead of the signal voltage of the common electrode elements, this complex signal voltage can therefore be supplied to the input of the amplifier 2S. In this case the separation stage 26 furnishes the signal S2 :and the separation stage 27 the `signal S1. This way of signal derivation is illustrated in FIG. 1 yby a broken line connecting the signal resistor 31 in the supply conductor to the anode 6 with the input of the amplifier 25.

The ldevice shown in FIG. 2 also serves to obtain two separate colour component signals. The device comprises a camera tube 5t? of the yorthicon type, comprising an electron gun 51, which is surrounded by an electron multiplier S2 for the return beam 77 in the 'ru-be. The ltube S0 is surrounded by the `conventional directional coil A53, the deflection coil 54 yand the focusing coil 55. Near the image window 56 of the tube is provided a target plate 57, which consists of a transparent, non-photo-conductive semi-conductor, for example of conductive glass, i.e. glass having an electrical resistivity of 109 to l012 ohm. cm. This target plate is coated on the side facing the electron gun 51, with a light-sensitive mosaic 58, the elements of which yare capable of `emitting electrons under the action of a radiation. These elements may consist, in known manner of, for example, antimony caesium.

The target plate is provided on the other side with .al plurality of parallel, line-shaped electrode elements 60' and 61, the direction of which is transverse to ythe plane of the drawing and which are alternately connected electrically to one another. The interconnected :elements 60 are connected to 'a `conductor 62, leading outside the tube and the interconnected elements 61 Iare connected to a `conductor 63, leading `to the outerside.

The electrode elements 60 and 61 are transparent and may consist each of, for example, a very thin layer of gold. To the target plate is added a two-colour filter, which has alternate paths of different spectra-l transparencies. These paths are arranged so that a radiation image formed by means of an optical system 64 on the mosaic plate 58 produces alternate paths each having one` of the colour components, the paths with one colour component being located opposite the electrode elements 60 `and the paths with the other colour component being located opposite the electrode elements 61. Such a. colour filter, similar to the colour l-ter consisting of the paths 12 and 13, in the camera tube of the device shown in FIG. l may 'be arranged on the side of the electrode elements facing the image window 56. As an alternative, the target plate 57 may be composed of differently coloured paths,

green paths 66 lying, for example, lbelow the electrode elements 6u and red paths 67 lying below :the electrode elements 6l. Asa further alternative, the radiation image may be produced irst with the aid of ian optical system at the area of a two-colour filter having red and green paths, this image being reproduced via the system 64 on the target plate 57.

Near in front of the mosaic layer 58 provision is made of a first net-shaped electrode 68, which is pervious -to electrons and which is connected to a supply conductor 69, leading outside the tube, whilst lat a slightly larger distance from the mosaic layer 58 a -second net-shaped electrode 74), pervious to electrons, is arranged and connected to a supply conductor 71, also leading outside the tube.

'The supply conductors 62, 63, 71 and 69 are connected-the latter via a signal resistor 73-to diiTerent points of -a potentiometer resistor 74, arranged over a voltage source 72. These points of connection are chosen so that the electrode elements 6u are at a positive bias voltage V1 relative to the cathode y65 of the electron gun 51 which cathode is connected to the negative terminal of the voltage source 72, and the electrode elements 61 are at a bias voltage V2, the net-shaped electrode 68 at a bias voltage V5 and the second net-shaped electrode '70 `at a bias Avoltage V6, so that V1 E V2 V5 and, moreover V6 V2. Herein E designates the first emission equilibrium voltage of the mosaic l-ayer 58; with antimony caesium mosaic elements this voltage may be about v. Practical values of 'the aforesaid bias voltages may then be, for example, V1-3 V., V2-7 v., V5-l0 v. and V5-5 v.

When the target plate 57 is scanned 'by an electron beam 76 emanating from the electr-on gun 51, those sections of the mosaic layer '5S which are located opposite the electrode elements 60 are stabilized on the potenti-al of the cathode 65. This is achieved by capturing electrons -from the beam 76. The excess of electrons of this beam return in the form of a return beam to the acceleration section of the tube i.e. to the electron multiplier 52, so that a signal voltage is produced across a signal resistor 78, which is included in the supply conductor to the llast anode 79 of the electron multiplier 52.

The parts of the mosaic layer 58 opposite the electrode elements `61 are stabilized, when scanned by the electron beam 76, on the potential of the net-shaped electrode 68 `and Ithis by secondary emission of the mosaic elements concerned. The secondary electrons have insuicient velocity to be able to pass through the second netshaped electr-ode 70 and are therefore collected by the tirst net-shaped electrode 68. Thus, across the signal resistor 73, is produced a Isignal voltage which is a measure Vfor the potential variation during a scaning period of -those parts `of the mosaic layer 58 which are located opposite the electrode elements 61.

Since the target plate 57 is conductive, the elements of the mosaic layer `58 will be subjected to a potential variation between two successive stabilisations also in darkness. Thus the potential of the mosaic elements opposite the electrode elements 60 increases, whereas the potential of the mosaic elements opposite the electrode elements 61 drops. These potential variations thus produce, already in darkness, signal voltages across the signal .resistors 73 and 78, these dark signals being, however, constant, so that they can be electrically compensated or separated oit.

If a radiation is projected, via the optical system 64 and the two colour ilter formed by the paths 66 and 67, onto the mosaic layer 58, the elements thereof will emit photo-electrons, in accordance with the local radiation intensity, so that during one image period the potential of these elements increases to a greater or smaller extent, this increase being superimposed on the potential variation yof the elements concerned, occurring already in the non-irradiated state. These increases inpotential therefore also become manifest in the ysignal voltages at the signal resistors 73 vand 78, these signal voltages being the higher, the stronger the mosaic elements concerned are irradiated.

rl'hesignal voltage at the resistor 73 is fed via a capacitor 80 to the input of an amplifier S1. Since the signal voltage at the resistor 73 is obtained, substantially only by the stabilisation of the mosaic elements opposite the electrode elements 61, these mosaic elements being irradiated, owing to the paths 67, mainly by that colour component of the radiation image which is allowed to pass through these paths, in this case red, the output signal S2 represents the .red colour component signal.

The signal voltage at the load resistor 78 of the last anode of the elect-ron multiplier 52 is fed -via a capacitor to the input of .a second amplifier 82. Since this signal voltage is obtained substantially only by the stabilisation of the mosaic elements opposite the electrode elements 60, which are irradiated via the paths 66 and therefore receive mainly the green colour component of the radiation image, the output signal S1 -of the amplifier 82 represents therefore the green colour component signal.

The device shown in FIG. 3 is intended `for producing three separa-te colour component signals S1, S2 and S3. The device comprises a camera tube V100, which, similar to the camera tube of the device shown in FIG. 1, comprises a photo-conductive target plate. Since a plurality of the elements `of the said camera tube 1 are also found in the tube 100, the corresponding elements of the latter are designated by the same reference numerals plus 1001. The reference numerals exceeding 135 designate elements which differ or do not occur in the device shown in FIG. l.

On the image window 104 of the tube 1130 is provided a three-colour filter consisting of cyclically successive red, green and blue paths, indicated by 112, 113 and 136 respectively. On each of these paths is provided a narrow, line-shaped electrode element, the electrode elements 114 on the paths `112 being electrically connected to a conductor 1.16, leading outside the tube, the electrode elements 115 on the paths 113 being connected to a conductor 117 leading outside the tube. Also the electrode elements 137 on the paths 136 are electrically connected to a conductor 138, leading to the outer side. However, a diierent way of connecting is possible for these elements, which will be evident from the description of FIGS. 4 and 5.

The electrode elements are coated by a photo-conductive target plate 105, which is constructed similarly to the target plate 5 of the camera tube of the device shown in fFIG. l. Near in front of the target plate provision is `made of a rst net-shaped electrode 110, which is connected to a conductor 11d, leading outside the tube. A second net-shaped electrode 139 is arranged at la slightly larger distance from the target plate 105 and is also provided with a conductor 140, also leading outside the tube.

The conductors 116, 117, 138, 140 and 111 are, the latter via a signal resistor 141, connected to different points of a voltage source 119, the negative terminal of which is connected to earth and also electrically to the earth-connected cathode 103 of the electron gun 102. Owing to these connections the electrode elements 114 have a positive bias'voltage V1, the electrode elements 115 a bias voltage V2, the electrode elements 137 a bias voltage V3, the first net-shaped electrode a bias voltage V5 and the second net-shaped electrode 139 a bias voltage V6, which are adjusted so that and, furthermore V1 V6 V5- The voltage E is the aforesaid irst emission equilibrium voltage of the material on the scanning side of the target plate 105. Practical values of the said bias voltages with an emission voltage E of about 30 v. are: V1-l5 v., V5-25 v., V2-45 v., V5-60 y. and V3-75 v.

The various bias voltages' result in that during operation when the target plate 105 is scanned by the electron beam 1232*, the surface parts of the target plate located opposite the electrode elements 114 are stabilized on the potential of the cathode 103, whereas the parts of thetarget plate provided with the electrode elements or the elements 137 are stabilized on the potential of the first net-shaped electrode 110. The stabilisation of the target sections provided with an electrode element 114 or 115 occurs similarly to the stabilisation of the target sections provided with an electrode element 14 or 15 in the camera tube of the device shown in FIG. l. The stabilisation ofthe target sections provided with an electrode element 137 is performed by capturing electrons from the electron beam 129. The velocity with which the electrons of this beam strike the target sections concerned is, indeed, sufficiently high to produce secondary emission with an em-ission coeicient of more than l, but owing to the positive bias voltage of the electrode elements 137 relative to the irst net-shaped electrode 110, these secondary electrons are nevertheless collected by the target sections concerned until these sectionsY are stabilized on the potential of the electrode 1.10. The correct stabilisations of the various target sections may be improved by providing the target sections opposite the electrode elements 114 with a very thin layer of material with low secondary emission,`for

example silver or gold, whereas, on the contrary, a readily secondary-emitting material, for example magnesia or cryolite may be arranged on the target sections opposite the electrode elements 115 and 137.

A radiation image formed on the target plate 1115 via the image window 104 reduces locally the high resistance of the target plate, which resistance is high in darkness, in accordance with the local intensity of the colour component allowed to pass through one of the paths 112, 113 and 136. This decrease in resistance results in that between two successive scans of the same point of the target plate the potential thereof varies to an extent varying with the local intensity of the radiation. As described in connection with the corresponding parts of the camera tube 1 of the device shown in FIG. l, the potential of those target sections which are opposite the electrode elements 114 with the bias voltage V1 increases and the potential of the sections opposite the electrode elements 115 with the bias voltage V2 decreases. The potential of those target sections which are opposite the electrode elements 137 will, however, decrease owing to the higher bias voltage V3 of these electrode elements.

The stabilisation of the target sections opposite the electrode elements 115 and 137 thus occurs by a potential leap which has opposite senses for the two kinds of elements. Thus signal voltages are obtained also in opposite senses, so that they can be separated. In the present case the two signal voltages are obtained, in the form of a complex voltage, in common from the signal resistor 141 in the supply conductor to the first net-shaped electrode 110. This is possible, since the electrons returned from the beam 129 after the stabilizing voltage has been reached at the target sections opposite the electrode elements 115 and 137, cannot pass the second net-shaped electrode 139, so that they are substantially all collected by the net-shaped electrode 110. The signal voltage at the resistor 141 is fed via a capacitor to the input of the amplier 125. The output voltage of this amplifier is fed to two separation stages 126 and 127 for opposite detection. The output voltage of the separation stage 127, which passes only the positive parts of the signal voltage which is common to the elect-rode elements 115 and 137, is formed by the colour component signal S, emanating from the stabilisation of the target sections opposite the electrode elements 137. The colour component associated With this signal is given by the spectral transparency of the paths 136 of the three-colour lter. The output signal S2 of the separation stage 126 emanates from the target sections opposite the electrode elements 115, the colour component associated with this signal is determined bythe spectral transparency of the paths 113.

The colour component signal Sl, associated with the colour component determined by the paths 112 of the three-colour filter, is formed by the output voltage of the amplifier 142, to the input of which is fed the signal Voltage across the resistor 131 in the supply conductor to the anode 106 of the electron gun 102. This signal voltage is determined by the return beam 128, which is formed by the electrons of the beam 129. These are the electrons which are returned after the target sections concerned have been stabilized on the cathode potential. These returned electrons pass, for the major part, through the second net-shaped electrode 139 and are collected by the anode 106 and, as the case may be, by the conductive wall coating 108 and the net-shaped electrode `1117 connected thereto.

In the camera tube 100 of the device shown in FIG. 3 the three groups of electrode elements, i.e. 114, 115 and 137 respectively, are provided each with a separate supply conductor, leading to the outer sides, 115, 117 and 11S respectively. However, as an alternative, only two of these groups, i.e. `the group with the highest and that with the lowest bias voltage, may be provided with such a supply conductor, whilst the desired bias voltage of the elements of the third group intermediate between the said bias voltages may be obtained by voltage divisionwith the aid of means provided in the interior of the tube. Two examples thereof are shown diagrammatically in FIGS. 4 and 5, which illustrate the image side of a camera tube, viewed from the electron gun.

According to FIG. 4 the electrode elements 114, 115 and 137 alternate cyclically, as it is the case with the tube 10i) of FIG. 3. Underneath the electrode elements are provided cyclically alternating, coloured paths (not shown) of a three-colour iilter. The lett-hand ends of the electrode elements 114 are electrically connected to one another by means of a metal strip 143, to which the supply conductor 116 is secured. The right-hand ends of the electrode elements 137 are similarly electrical-1y connected to one another by means of a metal strip 144, to which the supply conductor 13S is connected. It is evident from FIG. 3 that the elements 114 have, via the conductor 116, a bias voltage V1 and the elements 137, via the conductor 138, a'bias voltage V3. The desired bias voltage V2 of the elements 115 is now obtained by means of resistance path 145, extending transversely to all electrode elements, and contacting these elements, this path consisting for example of a semi-conductor, for example cadrniurnor Zinc `oxide or, if `desired of a very thin metal layer, for example of gold. With the aid of lthis path 145 only, the potential of the electrode elements 115 will lie substantially midway between the potentials of the elements 114 and 137. It is sometimes advantageous to have the potential of the elements differ slightly from this aver-age potential. 'Dhis may be achieved by connectin in addition, each of the elements 115 via an additional resistor to one or more elements 137 or 114. Such an additional resistor may be `formed by a resistance path 146, Iformed by, for example, a semi-conductor.

According to FIG. 4 each of the resistance paths 146 is short in order to avoid an electrical contact with the electrode elements 114. The resistance paths 146 may be caused to merge with `one another and hence to `form a single path, if either the electrode elements 114 terminate on the left-hand side of the said resistance path, or the electrode elements 114 and this resistance path are separated iby yan interposed insulating layer. Instead off using separate resistance paths 146, the parts of the resistance path extending 'between an electrode element 115 and an adjacent electrode element `137 may have a lower Iresistance than the other parts of the said path.

FIG. 5 shows a slightly rnodiiied order of succession of the electrode elements. Alternately there is an electrode 115, which is located on a green path (not s'ho'wn) of the three-colour filter. On one side of the element 115 there is always an electrode 114, arranged on a redV path and on the other side there is always an electrode element 137, arranged on a blue path. The electrode elements 114 are electrically connected to one'ano-ther and to the supply conductor 1'15 by the conductive strip 143, the electrode elements 137 are similarly connected by the oonductivestrip 144. All electrode elements are arranged transversely to and are in electrical contact with a resistance path 145, whilst only the electrode elements 115 and the elements 137 are each in electrical contact with a transverse resistance path 146. In the manner shown in FIGS. 4 and 5 -the electrode elements 115, without a separate supply conductor, have a bias voltage V2, which is nearer V3 than V1. Since it is not necessary to provide a supply conductor to be connected to a third group of electrode elements and leading outside the tube, the manufacture of -a tube for use in a device according to the invention for producing three separate colour component signals is more simple.

What is claimed is:

1. Means for converting a radiation image into color component signals comprising a camera tube having a radiation sensitive target plate of semicond-uctive material, la plurality of alternate electrode elements `on said target pla-te arranged in `at least two separate electrically connected groups, the target elements associated with said groups of electrode elements having different spectral response, a net-shaped electrode in front of said plate, means comprising a cathode for scanning the front of said plate with an electron beam, means applying a positive voltage on said net-shaped electrode relative to said cathode and exceeding the first emission equilibrium voltage of the material of said target, and means applying different positive bias voltages to said groups of electrodes, with one bias voltage being lower than said equilibrium voltage, and one bias voltage exceeding said first equilibtrium voltage but being lower than the voltage of said netshaped electrode.

2. The means of claim l in which said groups of target elements have different spectral sensitivities.

3. The means of Yclaim l in which means are provided for irradiating said Igroups of target elements with radiaftion of different spectral regions.

4. The means of claim jl in which said target plate is photo-conductive, with regions of said plate adjacent the electrode elements of only one of said groups having a 'high first emission equilibrium voltage.

5. The means of claim 1 in which said target plate is photo-conductive, with regions of said plate adjacent they electrode elements of only one of said groups having a low irst emission equilibrium voltage.

6. Means for converting a radiation image into color component signals comprising a camera tube having a radiation sensitive target plate of 'semiconductive material, a plurality o-f alternate electrode elements on said target plate arranged in at least two separate electrically connected groups, the target elements associated with said groups of electrode elements having different spectral response, a net-shaped electrode in front of said plate, means comprising a cathode for scanning the front of said plate with an electron beam, means applying `a positive voltage on said net-shaped electnode relative to said cathode and exceeding the first emission equilibrium voltage of the material of said target, and means applying different positive bias voltages to said groups of electrodes, with one bias yvoltage being lower than said equilibrium voltage, and one bias voltage exceeding said first equilibrium voltage but being lower than the voltage of said netshaped electrode, said plate having on the front side thereof a mosaic which emits photo-electrons upon irradiation, and collecting electrode means within said tube.

7. The means of claim 6 in which means are provided for deriving separate color component signals from the signal voltages at said groups of electrode elements and from said collecting electrode.

`8. The means of claim 6 in which a second net-shaped electrode yis providedV between the first-mentioned netshaped electrode and said cathode, said second net-shaped electrode having a potential less than the potential of said first net-shaped electrode, and means deriving separate color component signals from said first net-shaped electrode and said collecting electrode.

9. Means for converting a radiation image into color component signals comprising a camera tube having a radiation sensitive target plate of semiconductive material, a plurality of alternately disposed electrode elements on said target plate arranged in three separate electrically connected groups, the target elements associated with said groups of electrode elements having different spectral response, first and second net-shaped electrodes in front of said plate with said first net-shaped electrode being closer to said plate than said second net-shaped electrode, means comprising a cathode for scanning the front of said plate with an electron beam, means applying a positive voltage on said first net-shaped electrode relative to said cathode and exceeding the first emission equilibrium voltage of Ithe material of said target, means applying different positive bias voltages to said groups of electrodes, with one bias voltage being lower than said equilibrium voltage, one bias voltage exceeding said equilibrium voltage but being lower than the voltage of said first netshaped electrode, and one bias voltage exceeding the voltage of said first net-shaped electrode, and means applying a positive voltage on said second net-shaped electrode less than the voltage on said first net-shaped electrode.

10. The means of claim '9 in which said groups of target elements have different spectral sensitivities.

y l1. The means of claim 9 in which means are provided for irradiating said groups of target elements with radiation of different spectral regions.

l2. The means of claim 9 in which said target plate is photo-conductive with regions thereof adjacent the elec- -trode elements of only one of said groups having a high first emission equilibrium voltage.

13. The means of claim 9 in which said target plate is photo-conductive with regions thereof adjacent the electrode elements of at least one of said groups having a low first emission equilibrium voltage.

14. The means of claim 9 comprising a collector electrode in said tube, and means for deriving separate color component signals from said groups of electrode elements and said collector electrode.

15. The means of claim 9 comprising a collector electrode in said tube, and means for deriving separate color component signals from said first net-shaped electrode and said collector electrode.

16. The means of claim 9 in which Where V1 is the bias voltage of one group which is lower than the equilibrium voltage, V2 is the bias voltage of another group Which is greater than said equilibrium voltage but lower than the first net-shaped electrode voltage V5, and V3 is the bias Voltage of the remaining group which is greater than the voltage V5 of the first net-shaped electrode. 5

17. The means of claim 9 in which the means applying the positive bias voltage exceeding said equilibrium voltage but being lower than the voltage of said first netshaped electrode comprises resistive strip means in contact with the electrode elements of all of said groups.

VReferences Cited in the file of this patent UNITED STATES PATENTS 2,548,118 Morton et al Apr. 10, 1951 2,614,235 Forgue Oct. 14, 1952 2,689,271 Weimer Sept. 14, 1954 2,843,773 Wardley July 15, 1958 2,861,206 Fiore et al a Nov. 18, 1958 

1. MEANS FOR CONVERTING A RADIATION IMAGE INTO COLOR COMPONENT SIGNALS COMPRISING A CAMERA TUBE HAVING A RADIATION SENSITIVE TARGET PLATE OF SEMICONDUCTIVE MATERIAL, A PLURALITY OF ALTERNATE ELECTRODE ELEMENTS ON SAID TARGET PLATE ARRANGED IN AT LEAST TWO SEPARATE ELECTRICALLY CONNECTED GROUPS, THE TARGET ELEMENTS ASSOCIATED WITH SAID GROUPS OF ELECTRODE ELEMENTS HAVING DIFFERENT SPECTRAL RESPONSE, A NET-SHAPED ELECTRODE IN FRONT OF SAID PLATE, MEANS COMPRISING A CATHODE FOR SCANNING THE FRONT OF SAID PLATE WITH AN ELECTRON BEAM, MEANS APPLYING A POSITIVE VOLTAGE ON SAID NET-SHAPED ELECTRODE RELATIVE TO SAID CATHODE AND EXCEEDING THE FIRST EMISSION EQUILIBRIUM VOLTAGE OF THE MATERIAL OF SAID TARGET, AND MEANS APPLYING DIFFERENT POSITIVE BIAS VOLTAGES TO SAID GROUPS OF ELECTRODES, 